Many magnetooptic disks, of the so-called hard-sectored type, such hard-sectored disks usually include molded surface perturbations for indicating the sector boundaries. These surface perturbations also indicate sector addresses and other control information. Such surface perturbations intensity modulate a laser beam that impinges on the perturbations for providing a detectable readback signal. In the data portions of a magnetooptic (MO) disk, the surfaces are smooth, yielding a constant intensity reflected light and readback signal. The information recorded in the MO data portions is represented by remanent magnetization of either one or second polarity at the reading and recording surface. As is well known, a light reflected by a magnetooptic recording layer rotates the linear polarization of the impinging light beam in accordance with the remanent magnetization, creating p and s polarization components. The components are separated in a readback optical system, such that the p component impinges upon a first photodiode, while the s component impinges upon a second photodiode. The difference in signal between the two photodiodes is representative of the recorded information.
It is desired to use but one circuit to read both the intensity modulation and the MO modulated signals. In many differential detection circuits the intensity of the signal going on one diode can be made equal to the other diode, such that the differential signal is zero. Therefore, intensity modulation cannot be readily and reliably detected. In a similar vein, if there is intensity modulation of an MO signal, detection also becomes more difficult.